1. Field of the Invention
The present invention relates to a magnetic storage device for storing information in magneto-resistive elements.
2. Description of the Related Art
In recent years, MRAM (Magnetic Random Access Memory) has been receiving attention as a storage device for use in information processing devices such as computers and communication devices. With MRAM, data is stored magnetically, and the direction of magnetization can be maintained without using any electrical means. Volatile memory such as DRAM (Dynamic Random Access Memory) and SRAM (Static RAM) has the disadvantage that information is lost when power failure occurs. However, such a disadvantage can be avoided with MRAM. Furthermore, when compared with conventional non-volatile storage means such as flash EEPROM and hard disk devices, MRAM is superior in terms of access speed, reliability, power consumption, and the like. Therefore, it is believed that MRAM has functions which can replace all the functions of volatile memories such as DRAM and SRAM and of nonvolatile storage means such as flash EEPROM and hard disk devices (see the publication of Japanese Patent No. 3466470).
For example, in the development of information devices with the aim of realizing so-called ubiquitous computing which enables information processing at any location, storage devices are required which are adaptable to high-speed processing while power consumption is reduced, and in which loss of information can be avoided even when power failure occurs. MRAM has the potential to meet all these requirements simultaneously and is expected to be employed in a variety of information devices in the future.
In particular, sufficient power supply cannot always be provided in tablets, portable information terminals, and the like which are intended to be carried on a daily basis. Therefore, in order to carry out a large amount of information processing in severe use environments, a further reduction in power consumption during information processing is required even for MRAM, for which low power consumption is expected.
As examples of the technologies intended to further reduce power consumption in MRAM, Japanese Patent Application Laid-Open Nos. 2000-90658 and 2004-128430, for example, disclose magnetic storage devices. Each of these magnetic storage devices comprises: bit lines; word lines arranged orthogonal to the bit lines; tunneling magneto-resistive (TMR) elements; and the like. Each of the TMR elements is provided for each storage area (memory cell) and is arranged at the intersection of the corresponding bit line and the corresponding word line so as to be placed between the intersecting bit and word lines. Furthermore, in these magnetic storage devices, a yoke (being a magnetic field control layer) is arranged around each TMR element between the bit and word lines so as to surround these lines. The yoke is made of a high permeability ferromagnetic material, and functions to reduce the occurrence of magnetic flux leakage from the bit line or the word line and to concentrate the magnetic flux into the TMR element. Therefore, a magnetic field required for inverting the magnetization of the TMR element can be obtained even with low power consumption. Moreover, the magnetic flux can be concentrated into the TMR element.
In this instance, a TMR element includes: a first magnetic layer (being a magnetic sensing layer) in which the direction of magnetization is changed according to an external magnetic field; a second magnetic layer in which the direction of magnetization is fixed; and a non-magnetic insulating layer sandwiched between the first magnetic layer and the second magnetic layer. Binary data is stored in the TMR element by controlling the direction of magnetization of the first magnetic layer so as to be parallel or anti parallel to the direction of magnetization of the second magnetic layer.
However, in the TMR elements used in such magnetic storage devices, the influence of the diamagnetic fields generated there inside increases as the size thereof decreases, and thus the strength of the magnetic field for writing must be increased. Therefore, a contradiction arises in that write currents value be increased when the degree of integration of a magnetic storage device is increased. Consequently, it is generally considered that it is difficult to reduce the size of magnetic storage devices and to increase the recording density thereof.
Furthermore, the increase of write currents results in an increase in the size of transistors for controlling the conduction of the current. Therefore, a problem exists in that the degree of integration cannot be increased.